The present invention relates to a cathode-ray tube, and more specifically to a glass panel section of a cathode-ray tube.
Conventionally, in a cathode-ray tube 10 as shown in FIG. 1, a phosphor screen is formed on the inner surface of a faceplate 22 of a glass panel section 20, and a funnel section 30 with a deflection yoke device (not shown) on its outer periphery is sealed on a skirt 24 of the glass panel section 20. A neck 40 protrudes from the funnel section 30. An electron gun (not shown) for emitting electron beam is received in the neck 40. The glass panel section 20, the funnel section 30, and the neck 40 together constitute the envelope of the cathode-ray tube 10. The envelope is exhausted to a high vacuum.
In the prior art cathode-ray tube 10 of this type, as shown in FIG. 1, the inner and outer surfaces of the faceplate 22 of the glass panel section 20 are curved with a certain curvature so as to project outward. The corners of the faceplate 22 are also curved. Thus, the front view of the faceplate 22 is not rectangular, but rather rounded as a whole, as shown in FIGS. 1 and 2. If the radii of curvature of the inner surface of the faceplate 22 along the lateral axis (Y--Y), longitudinal axis (X--X) and diagonal axis (D--D) are Rsi, Rli and Rdi, respectively, and if those of the outer surface along these three axes are Rso, Rlo and Rdo, respectively, as shown in FIGS. 3A to 3C, the faceplate 22 is generally designed and manufactured in a manner such that Rsi=Rli=Rdi=Ri and Rso=Rlo=Rdo=Ro, wherein Ri and Ro are predetermined values.
The reason why the inner and outer surfaces and corners of the faceplate 22 are curved in the aforesaid manner is that the inside of the envelope of the cathode-ray tube is kept at a high vacuum. Therefore, a substantial inward stress attributed to the difference between the atmospheric pressure and the internal pressure of the envelope is applied to the central portion of the faceplate 22 and a substantial outward stress is applied to the peripheral portion of the faceplate 22. Accordingly, the envelope may possibly implode if it is subjected to a small impact or if glass, of which the envelope is made, has a flaw. In order to reduce the possibility of such implosion, the prior art faceplate 22 is generally rounded as a whole.
However, the faceplate 22 thus designed is considered injurious to the eyes of viewers. An ideal screen for the viewers' eyes has been found to be flat rectangular screen in which the ratio among the maximum effective dimensions perpendicular to the tube axis (Z--Z), which respectively correspond to the distance between the center of the inner surface and a peripheral portion along the lateral axis (Y--Y), that between the center of the inner surface and a peripheral portion along the longitudinal axis (X--X), and that between the center of the inner surface and a corner along the diagonal axis (D--D) is 3:4:5. The prior art round faceplate doesn't have the desired ratio between the three dimensions, and is regarded as unfit as a picture screen.
A 14-inch cathode-ray tube is designed so that Rsi=Rli=Rdi=Ri.apprxeq.551 mm and Rso=Rlo=Rdo=Ro.apprxeq.575 mm, while a 26-inch cathode-ray tube is designed so that Rsi=Rli=Rdi=Ri.apprxeq.1,034 mm and Rso=Rlo=Rdo=Ro.apprxeq.1,100 mm. If the maximum effective length of the faceplate 22 along its longitudinal axis (X--X), that of the faceplate 22 along lateral axis (Y--Y) and that of the faceplate 22 along diagonal axis (D--D) are 2Sl, 2Ss and 2Sd, respectively, the 14-inch cathode-ray tube is designed so that Sl.apprxeq.140.4 mm, Ss.apprxeq.105.3 mm and Sd.apprxeq.166.7 mm, while the 26-inch cathode-ray tube is designed so that Sl.apprxeq.263.9 mm, Ss.apprxeq.197.9 mm and Sd.apprxeq.313.2 mm. Thus, in the cathode-ray tubes of both these types, the ratio Ss:Sl:Sd is approximately 3:4:4.75.
The harmful visual effect and the fear of implosion can be removed by greatly thickening the faceplate 22. If the faceplate 22 is thickened, however, the cathode-ray tube will increase in weight and cost and will not be prefered practically in the point of the optical properties.